Tram trains and voltage changeover procedures.

[Justin Smith]

I was travelling between Rotherham and Doncaster yesterday and noticed all the electrification masts going in for the (severely delayed) tram train to Parkgate. The new tram trains appear to be dual voltage 25kV ac and 750 dc, is that correct ? If it is how do they change over ? On Thameslink they change over but that`s easier because it`s 3rd rail to OHL. They raise the collection shoe, then raise the pantograph (or vice verse). But if the current is coming from the same place (i.e. OHL) how does it work ? ! ?

 

[edwin_m]

The Rotherham tram-train will actually operate on 750V throughout its route including the Network Rail part. It has been specified as dual voltage, and the OLE on the Network Rail part is supposed to be easily convertible to 25kV, because at some stage in the life of the tram-trains it's quite likely that electrification will extend through Rotherham for other reasons.

In summary, a changeover section as used in overseas tram-trains includes a longer neutral section than usual. This causes the equipment on the vehicle to go into a mode where it awaits detection of voltage on the line, and the relevant switchgear is operated according to which voltage is detected. To reduce the risk of stranding in the neutral section, it may be on a slope so the tram-train can roll out, and/or it may be possible to energise it temporarily.

 

[Justin Smith]

The Rotherham tram-train will actually operate on 750V throughout its route including the Network Rail part. It has been specified as dual voltage, and the OLE on the Network Rail part is supposed to be easily convertible to 25kV, because at some stage in the life of the tram-trains it's quite likely that electrification will extend through Rotherham for other reasons.

In summary, a changeover section as used in overseas tram-trains includes a longer neutral section than usual. This causes the equipment on the vehicle to go into a mode where it awaits detection of voltage on the line, and the relevant switchgear is operated according to which voltage is detected. To reduce the risk of stranding in the neutral section, it may be on a slope so the tram-train can roll out, and/or it may be possible to energise it temporarily.

Thanks for that, very interesting. Would the slope be a different way for each direction (up and down line) ? That is to say the slope is always down in the direction of travel. If so, what would happen on single lines ? If the slope wasn`t always down in the direction of travel, what would happen if the tram (on the upward slope) stopped ? Would it have to reverse to take another run at it ? ! ?

 

[edwin_m]

Thanks for that, very interesting. Would the slope be a different way for each direction (up and down line) ? That is to say the slope is always down in the direction of travel. If so, what would happen on single lines ? If the slope wasn`t always down in the direction of travel, what would happen if the tram (on the upward slope) stopped ? Would it have to reverse to take another run at it ? ! ?

The slope is likely to be the same direction, ie "uphill" for one direction of running, so obviously can't be too steep. Quite a complicated bit of design, especially considering the tram-train joining the railway might have to stop at a signal somewhere in the same area.

 

[ComUtoR]

On Thameslink they change over but that`s easier because it`s 3rd rail to OHL.

It's both. At City your on DC, shoes down, no pantograph. At Farringdon it's Pantograph up on AC and you get DC so you have both AC and DC at the same time as the shoes are fixed.

They raise the collection shoe, then raise the pantograph (or vice verse). But if the current is coming from the same place (i.e. OHL) how does it work ? ! ?

319 and 700 shoes are fixed so do not get raised. There is a dual voltage alarm on a 319 as you hit Farringdon but the unit is in AC mode. Whatever is onboard under the bonnet is doing the magic to prevent any issues. When you select DC ist drops the pan.

Same with City. The shoes are down but you raise the pantograph and your on both AC and DC. Your on DC initially but switch to AC mode then raise the pan. Shoes and Pan are in contact right up until departing Farringdon.

 

[Justin Smith]

The slope is likely to be the same direction, ie "uphill" for one direction of running, so obviously can't be too steep. Quite a complicated bit of design, especially considering the tram-train joining the railway might have to stop at a signal somewhere in the same area.

So what would, on the uphill slope, if the train coasted to a stop (in the neutral section) before it reached the changed current section ? Would it not have to reverse (which I thought was a big no no on the railways) to regain power, then have to "take a run at" the neutral section ?

 

[edwin_m]

So what would, on the uphill slope, if the train coasted to a stop (in the neutral section) before it reached the changed current section ? Would it not have to reverse (which I thought was a big no no on the railways) to regain power, then have to "take a run at" the neutral section ?

It would have to set back, after obtaining permission from either the NR signaller or the tram controller or both depending which side of the signal it is. Driver would then go to the rear cab and release the brakes. Either a signal or an operating rule would prevent any following tram-trains drawing up right behind a stopped one in this section. A crossover nearby would be desirable so if the tram was unable to run on AC it could return to the tramway section to be parked up somewhere instead of blocking other services.

 

[apk55]

On continental Europe there are many similar transitions where there is a change from one voltage to another. As well as Germany where the train tram concept was developed for example France has lots of 1.5KV/25KV changeovers and multi voltage locos or units are common. How do these countries cope.

I would see it being no different from a neutral section on our AC lines (which these units which these units will cope with).

 

[zuriblue]

On continental Europe there are many similar transitions where there is a change from one voltage to another. As well as Germany where the train tram concept was developed for example France has lots of 1.5KV/25KV changeovers and multi voltage locos or units are common. How do these countries cope.

I would see it being no different from a neutral section on our AC lines (which these units which these units will cope with).

Iirc the tram-trains at Karlsruhe (where the tram-train concept was developed) change from the mainline voltage (15 KV 16 2/3 Hz) to the street running voltage (750VDC) on the fly at the Hb with a neutral section. The same pan is used.

At Basel SBB the 1st through platform between the SBB and SNCF parts (Gleis 5) can be switched between 15kv and 25kv. With the advent of multi-standard stock and also with Switzerland joining Schengen any of the through platforms now handle international services. Apart from the Italian trains (which have separate pans for AC and DC) the same pan is used for any voltage.

Remember years ago this was common in the UK on the GEML and Glasgow Suburban services where the voltage would be dropped from 25kv to 6.25kv in areas where there were low clearances. The change was done automatically using track magnets.

 

[SpacePhoenix]

In Melbourne Australia they have at least one level crossing bisected by a tram route, the trams running on a different voltage to the main line trains

 

[LesS]

In Melbourne Australia they have at least one level crossing bisected by a tram route, the trams running on a different voltage to the main line trains

Melbourne had 4. They are called "Tramway Squares". One was replaced with an underpass a couple of years ago, so there are 3 remaining. The crossings are controlled from signal boxes and the signalman changes the voltages on the squares manually. I find it fascinating to view the overhead wiring and the variety of insulators separating the various voltages.

 

[Justin Smith]

On continental Europe there are many similar transitions where there is a change from one voltage to another. As well as Germany where the train tram concept was developed for example France has lots of 1.5KV/25KV changeovers and multi voltage locos or units are common. How do these countries cope.

I would see it being no different from a neutral section on our AC lines (which these units which these units will cope with).

I think you`re right. Which begs another question, are neutral sections always on the flat (or a slight downhill in the normal direction of travel) ?

 

[ComUtoR]

http://www.railforums.co.uk/showthread.php?t=134509&highlight=neutral+sections

This should help. Its all about Neutral sections

 

[D1009]

I think you`re right. Which begs another question, are neutral sections always on the flat (or a slight downhill in the normal direction of travel) ?

Neutral sections are sufficiently short under normal circumstances to enable trains to coast through them whether going uphill, downhill or on the flat.

 

[edwin_m]

Remember years ago this was common in the UK on the GEML and Glasgow Suburban services where the voltage would be dropped from 25kv to 6.25kv in areas where there were low clearances. The change was done automatically using track magnets.

The track magnets are still used on AC neutral sections. The difference is that the dual-voltage rolling stock was equipped with a circuit to detect what voltage was present, and would re-configure the transformer accordingly before closing the circuit breaker. This went wrong a few times in the early days, with dramatic consequences, but as far as I know was pretty reliable later on.

 

[zuriblue]

In Melbourne Australia they have at least one level crossing bisected by a tram route, the trams running on a different voltage to the main line trains

In Suhr, Kanton Aargau there is a crossing between the WSB narrow gauge line from Aarau to Menziken (750VDC) and the SBB line between Lenzburg and Zofingen (15 KV 16.7 Hz)

There was talk of extending one of the trolleybus routes in Zürich over the level crossing at Affoltern (600VDC/ 15KV) but they decided aginst it.

 

[edwin_m]

Porta Maggiore in Rome has crossings of tram lines and narrow gauge rail lines with a different overhead line voltage. As far as I could tell the section of OLE around the intersection is insulated in all four directions and can be switched to either voltage, presumably by the signalling system.

 

[boxy321]

Surely the two different supplies are simply OR'd together?

The 25kV is stepped down with a transformer and then rectified to make the DC voltage for the motors. The only issue would be if the HV section back-fed into into a 3rd rail if both supplies were there at once. Making the the transformer output slightly less than the DC power would prevent this.

This obviously isn't the case if two different voltages are available from the OLE>.

 

[edwin_m]

Surely the two different supplies are simply OR'd together?

The 25kV is stepped down with a transformer and then rectified to make the DC voltage for the motors. The only issue would be if the HV section back-fed into into a 3rd rail if both supplies were there at once. Making the the transformer output slightly less than the DC power would prevent this.

This obviously isn't the case if two different voltages are available from the OLE>.

That is moreorless what class 313s do though I think there is a circuit breaker so the shoes aren't live when running on OLE. However the tram-train takes both voltages through the same pantograph so has to do something a bit more complicated. Also I believe that the circuit breaker that switches the 25kV couldn't cope with the current if it ever tried to open on 750V, and the one that switches 750V couldn't cope with the voltage if 25kV ever got too near it.